WO2006048934A1

WO2006048934A1 - Link-adaptation system in mimo-ofdm system, and method therefor

Info

Publication number: WO2006048934A1
Application number: PCT/JP2004/016343
Authority: WO
Inventors: Lee Ying Loh; Choo Eng Yap
Original assignee: Matsushita Electric Industrial Co., Ltd.
Priority date: 2004-11-04
Filing date: 2004-11-04
Publication date: 2006-05-11
Also published as: JPWO2006048934A1; US20090067528A1; CN101053190A; EP1802017A1

Abstract

A link-adaptation system in a MIMO-OFDM system. In this system, a V-BLAST processing unit (308) executes a V-BLAST operation to separate a received signal into data streams in a manner to correspond to a plurality of antennas of a sender. A vector information output unit (312) sends the feedback-vector information obtained by the V-BLAST operation, to the sender. On the basis of the feedback-vector information, an adaptive bit assignment unit (304) controls the number of bits to be assigned, adaptively for every sub-carriers sent through different antennas. On the basis of the feedback-vector information, an adaptive power assignment unit (306) assigns the power adaptively to the individual antennas.

Description

Specification

MIMO—Link Adaptation System and Method in OFDM System

Technical field

[0001] The present invention relates to a link adaptation system and method in a multiple-input multiple-output (MIMO) communication system using orthogonal wave frequency division multiplexing (OFDM). Background art

[0002] Simultaneous transmission of multiple data streams involves multiple (N) transmit antennas and multiple (N) receive

T R

It is implemented in MIMO communication systems that use antennas. The signal travels through multiple paths from the transmitting antenna and is reflected and diffused before reaching the receiving antenna. An important feature of the Ml MO system is its ability to take advantage of multipath propagation and turn it into user convenience. One of these advantages is the increase in system capacity using spatial multiplexing, which is usually achieved by transmitting independent data over separate transmission links.

A well-known technique for increasing the data rate by spatial multiplexing is discussed in Non-Patent Document 1.

[0004] MIMO techniques were originally designed for narrowband wireless systems, ie, flat'fading * channels. Therefore, it is difficult to obtain a high effect in the frequency selective channel. Therefore, OFDM has been used in conjunction with MIMO systems to overcome the frequency selective channel proposed in the wireless environment!

[0005] Using Inverse Fast Fourier Transform (IFFT), OFDM can transform a frequency selective channel into a set of independent sub-channels of parallel frequencies. Since the frequencies of these subchannels are orthogonal and overlap each other, spectrum efficiency is improved and intercarrier interference is minimized. Multipath effects are further reduced by attaching a cyclic prefix to the OFDM symbol.

[0006] A plurality of communication channels existing between a transmission antenna and a reception antenna are usually changed over time. Each link state becomes different. A MIMO system with feedback provides channel state information (CSI) to the transmitter and allows the use of methods such as link adaptation or water filling to provide a higher level of performance.

[0007] Adaptive bit loading in OFDM systems has been discussed in various technical papers. By changing the number of bits allocated to OFDM subcarriers, adaptive 'bit' loading has the objective of optimizing the data rate without degrading system quality. This technique works on the fact that each different subcarrier has a variable attenuation depending on the channel conditions. Allocation decisions are usually made using specific feedback information such as channel state information (CSI) and signal-to-noise ratio (SNR) for each subcarrier.

[0008] Another example of link adaptation is the adaptive modulation Z code scheme (AMC). In conventional systems, the transmitter determines the modulation and coding scheme (MCS) level to use from a predefined set of levels. This determination is usually made by comparing the post-detection SNR measured at the receiver with the threshold and value corresponding to each MCS level. This method has a high processing complexity because the power SNR must be calculated for each received symbol, which is accurate in selecting the MCS level. In addition, the overhead of signal feedback is also pressing down on the limited radio resources available.

[0009] Processing complexity and high feedback A technique for reducing overhead is presented in Patent Document 1. In the technique described in the above document, the MCS level is determined based on _an acknowledgment ( _ACK ) and a negative acknowledgment (NACK) together with a specific periodic feedback message.

Non-Patent Literature 1: V-BLAST: an architecture for realising very high data rates over the rich-scattering wireless channel "by PW Wolniansky et al in the published papers of the 1998 URSI International Symposium on Signals, Systems and Electronics, Pisa, Italy , Sep. 29 to Oct. 2, 1998.

Special Hundred Literature: A mobile communication system and method for adaptive modulation and coding, combining pilot-aided and ACK / NACK based decision on the employed modulation and coding scheme level "by Cho Myeon— gyunand Kim Ho— jin, patent number EP1289181, filed 5thMarch 2003.

Disclosure of the invention

Problems to be solved by the invention

[0010] However, a technique for applying link adaptation to a MIMO-OFDM system is not disclosed.

An object of the present invention is to provide a link adaptation system capable of performing optimum bit allocation while suppressing processing complexity and reducing signal feedback overhead in a MIMO-OFDM system and its Is to provide a method.

Means for solving the problem

[0012] The present invention adaptively controls the number of bits allocated to each subcarrier transmitted by different antennas and the power of each transmission antenna based on feedback information provided by the V-BLAST processing unit on the receiver side. To do. Further, according to the present invention, on the receiver side, depending on the ACK / NACK information, an AMC level increase / decrease in each transmission antenna is determined in the next frame transmission, and obtained by V-BLAST processing. Depending on a set of link quality information, the AMC level increase Z decrease amount is determined.

The invention's effect

[0013] According to the present invention, in a MIMO-OFDM system, link adaptation can be performed while processing complexity is reduced and signal feedback overhead is reduced.

Brief Description of Drawings

[Fig. 1] ΜΙΜΟ—Block diagram of transmitter in OFDM communication system

[Fig.2] MIMO—Block diagram of receiver in OFDM communication system

FIG. 3 is a diagram showing an embodiment of a closed loop system employed for the purpose of link adaptation according to the present invention.

FIG. 4 is a flowchart showing a V-BLAST signal processing method and information acquisition in an embodiment of the present invention.

[Figure 5] The detection processing performance of each transmission layer on the assumption that error propagation does not occur Illustration

FIG. 6 is a diagram showing another embodiment of a closed loop system employed for the purpose of link adaptation of the present invention.

[Fig. 7] Diagram showing examples of combinations of different modulation schemes and code rate at 7 AMC levels

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram of a transmitter 100 in a multiple-input multiple-output communication system (that is, MIMO-OFDM system) using orthogonal wave frequency division multiplexing. FIG. 2 is a block diagram of the receiver 200 of the same system. Both figures show the power of a system that uses two transmit antennas and two receive antennas. The present invention uses multiple (N) transmit antennas and multiple (N

T

) Can be extended to a system using a receiving antenna.

R

In transmitter 100, data processing is performed for each individual antenna chain. Different streams of independent data are transmitted from each transmit antenna. First, a cyclic redundancy check (CRC) code is added to the input data by the CRC adding unit 102. Thereafter, channel code keys such as convolutional coding and turbo code keys are executed in the code key unit 104. The encoded data is then interleaved with interleaver 106 to reduce burst errors in the data. Multi-level modulation constellation 'symbol mapping is executed by the mapping unit 108 on the interleaved data. A pilot signal is inserted in pilot insertion section 110 with respect to the mapped signal. Inserting a no-lot signal facilitates channel evaluation at the receiver.

[0018] Before performing OFDM modulation, the SZP converter 112 converts a serial data stream into a parallel data stream. IFFT section 114 makes generated subcarriers orthogonal to each other. After the parallel data is converted into serial data by the PZS converter 116, a cyclic prefix for reducing the multipath effect is added to the OFD M symbol by the CP adder 118. Prior to transmission, the digital signal is converted to an analog signal by the DZ A converter 120. After various processing in each transmitter chain, the signal is ready to be transmitted by its assigned transmitting antenna 122. [0019] In receiver 200, the received signal from receiving antenna 202 is processed in the reverse manner, that is, conversion from analog to digital (AZD converter 204), cyclic prefix removal (CP Processing such as a removal unit 206), serial-parallel conversion (SZP conversion unit 208), fast Fourier transform (FFT unit 210), and parallel-serial conversion (PZS conversion unit 212) are performed. Since the received signal has a signal strength that overlaps multiple transmit antenna forces, it is necessary to separate this signal into separate streams. In this case, a zero-forcing (ZF) or minimum mean square error (MMSE) t, a V-BLAST decoder 214 that utilizes the technique is used to perform this function.

[0020] Further, after performing demapping (demapping unit 216), dingtering (Dinterleaver 218), and decoding (decoding unit 220), cyclic redundancy check (CRC processing unit 222) is performed on each packet. Confirm that the data is correct. If the inspected bucket is determined to be error free, an acknowledgment (ACK) is sent to the transmitter and the transmitter does not retransmit the packet. If there is an error, a negative response (NACK) is sent to the transmitter 100 to request retransmission.

FIG. 3 is a diagram showing a closed loop system adopted for the purpose of link adaptation of the present invention. The system shown in FIG. 3 employs a configuration including an adaptive bit allocation unit 304 and an adaptive power allocation unit 306 on the transmitter side.

The input data is turbo-coded by a turbo code key unit 302. The systematic bits and parity bits generated by the turbo code are output to the adaptive bit allocation unit 304.

Adaptive bit allocation section 304 adaptively controls the number of bits to be allocated for each subcarrier transmitted by different antennas. The adaptive power allocation unit 306 adaptively allocates power to each antenna. The number of allocated bits and the amount of power depend on the previous transmit power obtained antenna state. This information is provided by the V-BLAST processing unit 308, stored in the vector information output unit 312, and sent to the transmitter through the error free channel 310.

[0024] V-BLAST processing section 308 performs V-BLAST processing for separating a received signal into a data stream corresponding to a plurality of antennas of the transmitter. The demapping unit 314 demaps each bit subjected to V—BLAST processing. FIG. 4 is a flowchart showing a V-BLAST signal processing method and information acquisition in an embodiment of the present invention. Based on the information obtained from the V—BLAST process, the antenna SNRs are ranked. The V-BLAST technique using the ZF criterion and the antenna ranking procedure are described below.

[0026] The signal received by each receiving antenna is composed of a mixture of signals from each transmitting antenna. Therefore, V-BLAST aims to detect hybrids and separate them into valid data streams. The V-BLAST technique is performed for each sequence of symbols obtained from all received antenna power (this is called the received vector). The channel matrix obtained from the channel evaluation corresponding to each received vector is also required for V-BLAST processing.

[0027] The first step after setting the received vector (r) and the corresponding channel matrix (H) set is to calculate the pseudo inverse of H to obtain the set of matrices G (step 404). ).

[0028] After obtaining the pseudo inverse of the channel matrix, the norm of each row of G is calculated (step 406).

The purpose of this step is to select the first detection target layer. It has already been proven that better performance can be achieved if detection is performed first at the layer with the highest signal-to-noise ratio (SNR) after detection. This is equivalent to detecting the row of G with the minimum norm.

[0029] Therefore, select the row of G with the minimum norm as the zeroization vector (w) (step

i k

408). This vector zeros (disables) all signals except those transmitted at k ^th . The V-BLAST processing unit 308 internally generates a reception vector (r) and a weight vector (w).

i k

As a result, the symbols transmitted from the k ^th transmission antenna are detected (step 410).

[0030] The detected symbols are remapped by slicing to the nearest value in the signal constellation (step 412). In this way, the evaluation for signal cancellation performed in the next step is efficiently processed.

If one layer is detected, subsequent layer detection processing can be performed efficiently. By subtracting the vector force of the received signal from the detected signal part (step 414), the number of layers to be zeroed in the next step is reduced. This erasure process corrects the received signal vector to a vector with a reduced interference signal component. It also reduces detection complexity. [0032] Corresponding to this, by deleting the k ^th column (step 416), the channel matrix also needs to be corrected.

[0033] The entire process is repeated until all layers are successfully detected (step 418). Note that V-BLAST detection continues for other sets of received vectors.

[0034] Since the norm is an indication value of SNR after detection of each transmission layer, the present invention uses this information for antenna ranking. The order of detection for each symbol in the frame is stored as a vector in the receiver. The receiver then feeds back the information vector to the transmitter through an error-free 'channel. The transmitter uses this vector for link adaptation in the next transmission frame. This storage and feedback process is repeated at the desired frequency. These two steps may be performed for each frame received at the receiver or may be performed for every desired m frames. As a general guideline, feedback is performed in a shorter period for a radio channel that fluctuates quickly. On the other hand, slow-changing channels should update information at longer intervals to save resources and keep complexity at a minimum level.

FIG. 5 is a diagram showing the performance of each detected transmission layer on the assumption that error propagation does not occur during V-BLAST detection. In Fig. 5, the horizontal axis is E ZN and the vertical axis is BER (b O

Bit Error Rate). In Fig. 5, communication is performed between a transmitter with four transmitting antennas and a receiver with four receiving antennas, and the signals of transmitting antenna 4 (Tx4) are separated in order from the signal of transmitting antenna 1 (Txl). Show the error curve for the case.

Theoretically, the diversity level is lowest when the first detection target layer is detected. For each detected layer, each received antenna is constant, but the detected layer is erased, and as a result, the diversity level of the system increases as the layer progresses. This is clearly shown in FIG. The error curve of antenna 1 detected in the first decoding 'step gradually falls as the SNR increases (diversity level of antenna 1). The error curves for antennas 2, 3, and 4 drop much more rapidly than for antenna 1. Earlier symbols are subtracted and gained, increasing the diversity level power in subsequent layers. Level 4 occurs when it is detected by four receiving antennas. This is the case when only one signal remains. Therefore, it can be concluded from this observation that the first detection target layer after ranking has the highest post-detection SNR, but its performance is lower than other transmission layers detected after that. That is. Therefore, the adaptive bit allocation unit 304 and the adaptive power allocation unit 306 can reduce the number of bits for the antenna that has become the first detection target layer to improve the performance in this specific antenna link. High transmit power or both should be provided.

[0037] Another embodiment of the present invention is shown in FIG. In this embodiment, AMC is used. V—In addition to BLAST information, the system uses CRC information to facilitate AMC processing. Multiple CRC adding section 604 in transmitter 602 adds a CRC bit to the signal transmitted from each antenna chain 608. Therefore, in the receiver 612, each frame received by each individual reception antenna and demodulated by the demodulation decoder 616 is subjected to CRC for error detection in the multiplexed CRC detection unit 618.

[0038] AMC selection section 624 obtains a CRC result (ACKZNACK) from multiplexed CRC detection section 618. The AMC selection unit 624 determines whether to increase or decrease the AMC level in each antenna 608 of the transmitter 602 in the next frame transmission according to the ACKZ NACK information for the signal received by each reception antenna 610. The AMC selection unit 624 increases the AMC level in the next frame transmission for the antenna that has received the ACK in the current transmission, and decreases the AMC level in the next frame transmission for the antenna that has received the NACK. .

In addition, the AMC selection unit 624 determines an increase Z decrease amount of the AMC level depending on a set of link quality information obtained by the V-BLAST decoder 614. Generated norm information force Used as a means to determine the link state evaluation of each antenna. By using the antenna ranking concept described above, the AMC selector 624 increases the AMC level significantly for antennas with relatively low error probabilities, and when the AMC level needs to be reduced. For antennas with a relatively high error probability, the AMC level is greatly reduced.

[0040] Based on both the ARQ and link quality information, the AMC selector 624 determines whether the appropriate AMC Select the bell and feed back this selection through feedback channel 626. It should be noted here that all computation and selection processes are performed at the receiver. As a result, the overhead and amount of signal information that needs to be sent to the transmitter is reduced.

Using this feedback information, multiplexing AMC unit 606 in transmitter 602 can efficiently assign an AMC level to each transmission antenna. By assigning the appropriate AMC level to each antenna based on the link quality determined during the previous transmission, the system can better utilize the channel state differences and variations to improve overall system performance. It is out.

[0042] SNR measuring section 622 measures the SNR using the received pilot signal transmitted from each transmitting antenna 608. AMC selection section 624 evaluates the channel state of each transmitting antenna 608 based on the measured SNR, and periodically resets the AMC level according to the SNR after detecting the pilot signal transmitted as the reference value. In addition, the AMC selection unit 624 executes allocation based on the AMC level SNR in the initial setting period. This prevents the actual AMC level range power that the system can support from deviating too much from the AMC level assigned by the above method.

FIG. 7 shows an example of AMC level diversification. Here, there are seven levels, the modulation method varies from QPSK to 64QAM, and the coding rate varies from 1/4, 1/2, and 3/4. The higher the AMC level assigned, the higher the data rate. To obtain a high data rate while keeping the error rate within acceptable limits, the signal's SNR should always be above the threshold corresponding to each AMC level.

Note that the above description is considered to be a preferred embodiment of the present invention. The present invention is not limited to the disclosed embodiment, and can be implemented in various forms and embodiments. The scope is determined with reference to the following claims and their equivalents.

Industrial applicability

The present invention is suitable for use in a multiple-input multiple-output (MIMO) communication system using orthogonal wave frequency division multiplexing (OFDM).

Claims

The scope of the claims

[1] A link adaptation system in a multiple input multiple output (MIMO) communication system using orthogonal wave frequency division multiplexing (OFDM),

The receiver separates the received signal into data streams corresponding to the multiple antennas of the transmitter V-BLAST signal processing unit that performs B-BLAST processing, and feedback “vector information obtained by V-BLAST processing is transmitted. A bit allocation for adaptively controlling the number of bits allocated by the transmitter based on the feedback vector information for each subcarrier transmitted by a different antenna. A part.

[2] The system according to claim 1,

The transmitter further includes a power allocation unit that adaptively allocates power to each antenna based on the feedback vector information.

[3] The system according to claim 1,

The receiver sends information indicating the antenna position having the highest post-detection SNR as the feedback vector information to the transmitter;

The bit allocation unit of the transmitter allocates a smaller number of bits to the antenna of the feedback vector information.

[4] The system according to claim 2,

The power allocation capacity of the transmitter allocates higher transmission power to the antenna of the feedback vector information.

[5] A link adaptation system in a multiple input multiple output (MIMO) communication system using orthogonal wave frequency division multiplexing (OFDM),

The receiver separates the received signal into data streams corresponding to the multiple antennas of the transmitter, V-BLAST signal processing unit for performing BLAST processing, CRC detection unit for performing error detection for each data stream, For each transmitter antenna of the transmitter, an adaptive modulation Z coding system (AMC) level increase / decrease is determined depending on the error detection result, and V-BLAST processing is performed. An AMC selection unit that determines an increase or decrease in AMC level depending on link quality information obtained, and

The transmitter includes: a CRC adding unit that adds an error detection bit to a transmission signal; and a multiple AMC unit that assigns an AMC to each transmission antenna based on a result of the AMC selection unit.

[6] The system according to claim 5,

The AMC selector of the receiver raises the AMC level in the next frame transmission for the antenna that received the acknowledgment (ACK) in the current transmission, and the next in the antenna that received the negative acknowledgment (NACK). The AMC level is lowered by sending a frame.

[7] The system according to claim 5,

AMC selection power of the receiver When the AMC level needs to be increased, the error probability is relatively small, the antenna increases its AMC level more, and when the AMC level needs to be decreased, the error probability is relatively low. Larger antennas reduce their AMC level more.

[8] The system according to claim 5,

The receiver further includes an SNR measurement unit that measures a signal-to-noise ratio (SNR) using a received pilot signal transmitted from each transmitting antenna of the transmitter;

The AMC selection unit evaluates the channel state of each transmit antenna based on the measured SNR, and periodically resets the AMC level according to the SNR.

[9] A link adaptation method in a multiple-input multiple-output (MIMO) communication system using orthogonal wave frequency division multiplexing (OFDM),

In the receiver, performing a V-BLAST process that separates the received signal into data streams corresponding to the multiple antennas of the transmitter; sending feedback vector information obtained by the V-BLAST process to the transmitter; Comprising

In the transmitter, the method includes adaptively controlling the number of bits to be allocated based on the feedback vector information for each subcarrier transmitted by different antennas.

[10] The method of claim 9,

The transmitter further includes adaptively allocating power to each antenna based on the feedback vector information.

[11] The method of claim 9,

In the receiver, information indicating the antenna position having the highest post-detection SNR as the feedback vector information is sent to the transmitter,

In the transmitter, a smaller number of bits is allocated to the antenna of the feedback vector information.

[12] The method of claim 10,

In the transmitter, higher transmission power is allocated to the antenna of the feedback vector information.

[13] A link adaptation method in a multiple-input multiple-output (MIMO) communication system using orthogonal wave frequency division multiplexing (OFDM),

At the receiver, a step of performing a V-BLAST process for separating a received signal into a data stream corresponding to a plurality of antennas of the transmitter, a step of performing error detection for each data stream, and a step of transmitting each of the transmission antennas of the transmitter Then, the adaptive modulation Z coding (AMC) level is increased or decreased depending on the error detection result, and the AMC level increase or decrease is decreased depending on the link quality information obtained by V-BLAST processing. A step for determining, and

The transmitter includes a step of adding an error detection bit to a transmission signal and a step of assigning an AMC level to each transmission antenna based on the determination result of the AMC level.

[14] The method of claim 13,

The receiver receives the acknowledgment (ACK) in the current transmission and raises its AMC level in the next frame transmission, and the antenna receives the negative acknowledgment (NACK) in the next frame. Reduce the AMC level on transmission.

[15] The method of claim 13,

In the receiver, if the AMC level needs to be increased, the error probability is relatively small, the antenna increases the AMC level more, and if the AMC level needs to be decreased, the error probability is relatively large. The AMC level is further reduced.

[16] The method of claim 13,

Further comprising the step of measuring a signal-to-noise ratio (SNR) using a received pilot signal transmitted from each transmitting antenna cable of the transmitter to the receiver;

The channel state of each transmit antenna is evaluated based on the measured SNR, and the AMC level is reset periodically according to the SNR.